This invention generally relates to an ion implant apparatus which is used in a process of manufacturing a device such as a semiconductor device and, more particularly, to an ion implant apparatus which prevents a semiconductor wafer from being charged, i.e., prevents the insulator of the semiconductor wafer from storing positive charges, when positive ions are implanted in the wafer.
Recently the ion implant apparatus has been widely used to introduce impurity ions into a target such as a semiconductor wafer (hereinafter called "wafer"). Advantageously the ion implant apparatus is capable of controlling the number of the implanted dopant atoms and the depth of the implantation with high precision. Because of this advantage, the ion implant apparatus has become indispensable to the introduction of impurity ions into the wafer.
Generally in the ion implant apparatus, positive ions are accelerated into a wafer for doping. When accelerated positive ions hit the wafer, secondary electrons are emitted out of the wafer, or positive charges are stored in the insulator. Consequently the surface of the wafer becomes inclined to be charged positively, which is a factor for causing electrostatic breakdown to the insulator of a semiconductor device. This results in dropping of productivity.
In view of the disadvantage of the ion implant apparatus, it is essential to prevent the wafer from being charged positively in ion implantation process. The prior art apparatus for avoiding the positive charging of the wafer is the electron flood system shown in FIG. 6.
As shown in FIG. 6, the prior art electron flood system is to prevent the wafer from being charged positively. Positive ions 10 to be implanted are radiated to a wafer 11 on a disk 12 through an ion beam introduction tube 13. While radiating, a current is applied to a filament 14 to generate thermoelectrons (hereinafter called "primary electrons"). The primary electrons are accelerated by a voltage of a primary electron accelerating power source 17 into a primary electron beam 16. The primary electron beam 16 is caused to collide with the inner wall of the ion beam introduction tube 13 so as to generate electrons with lower energies (hereinafter called "secondary electron").
The secondary electrons are trapped in the ion beam 10 to be transported to the wafer 11. Thus the wafer 11 which lacks electrons is supplied with the secondary electrons thereby to prevent the wafer 11 from being charged positively. The filament 14 radiates the primary electrons to the entire circumference, a primary electron reflecting plate 15 is provided at the back of the filament 14 for reflecting the primary electrons, and a primary electron reflecting power source 18 supplies the primary electron reflecting plate 15 with a reverse bias voltage. This makes it possible to use almost all of the primary electrons generated as a primary electron beam 16.
But the prior art apparatus has the following disadvantage. The secondary electrons with an energy about equal to or less than 10 eV which are most effective to prevent the wafer from being charged are effectively generated. But the electrons with an energy more than 20.about.30 eV, which are a part of the secondary electrons and the primary electrons are also transported to the wafer 11. Consequently the wafer 11 is supplied with electrons in a number exceeding a required number, the wafer is charged negatively, and the insulator of the semiconductor wafer suffers electrostatic breakdown.
Furthermore the prior art apparatus has the following disadvantage. An ampere meter 20 measures the current of the ion beam 10. The ampere meter 20 is connected to the ion beam introduction tube 13 and the disk 12. Some of the primary and secondary electrons are driven out of the system of measurement of the ampere meter 20. These electrons cause errors in the ampere meter 20. These errors depend on the leakage of electrons out of the system.
The following is still another disadvantage of the prior art apparatus. Since the ion beam introduction tube 13 is subjected to the radiation of the ion beam 10 passing therethrough, ions stay on the inner wall of the tube 13 as impurities. In the prior art device of the structure as shown in FIG. 4, the ion introduction tube 13 becomes very hot due to the exothermic generated when the primary electron beam is radiated to the ion beam introduction tube 13. This frees or sputters the impurities staying on the tube 13, consequently polluting the wafer 11.